"One or more addons do not have UTF-8 encoding"
Even if you solve the UTF-8 Warning, the other warnings will not disappear -- but there's no reason to worry about them: here's why:

The warning icons to the right of those add-ons are benign and intentionally added by the authors of those add-ons to warn users about something -- but they have always been there.

It's been pointed out above, but it's worth a mention and a screenshot.



Addons can define if they should be displayed with warning icons, this is to indicate to the user that there's some remaining work to be done on the addons. The message about the UTF-8 isn't caused by something that also causes those warnings. I have them too and my Blender works fine, and those warnings don't affect any part of Blender other than to give that indication.

The fact that the UTF-8 warning uses the same icon doesn't mean that it's the cause of those warning icons on the other addons, though it's understandable that you might think so. This is merely a matter of 'icon reuse'

Take for instance the Render: Network Render Addon. An addon you are not likely to ever use unless you have a network. In its source code (in its __init__.py file) it has a short manifest which we call a bl_info dictionary. It looks like this:

bl_info = {
    "name": "Network Renderer",
    "author": "Martin Poirier",
    "version": (1, 8),
    "blender": (2, 60, 0),
    "location": "Render &gt; Engine &gt; Network Render",
    "description": "Distributed rendering for Blender",
    "warning": "Stable but still work in progress",
    "wiki_url": "http://wiki.blender.org/index.php/Doc:2.5/Manual/"
                "Render/Engines/Netrender",
    "category": "Render",
}


The bl_info is a way for an add-on to tell Blender 'Hey! add me to the add-on list, and display this information about me`. Without that bl_info add-ons will not appear in the add-ons list.

The inclusion of the key / value pair "Warning":  "Stable but still work in progress" is the sole reason you see the warning icon beside that addon in your preferences -> Add-ons tab. All the other addons in that list that have the warning icon, has a similar entry in their bl_info.